1. Field of the Invention
The invention relates to a die cavity of a continuous casting die.
2. Description of Related Art
Continuously cast long products are predominantly cast in tubular casting dies with a rectangular, in particular with an approximately square or round cross-section. The billets and blooms are then further processed by rolling or forging.
Uniform heat transfer along the peripheral line of the strand cross-section between the strand being formed and the die cavity wall is of vital significance to the production of continuously cast products, especially of billets and blooms, having good superficial and microstructural quality. Many proposals are known for configuring the die cavity geometry, in particular in the region of the concave corner surfaces of the die cavity, in such a manner that no air gaps occur between the strand shell being formed and the die wall which cause reheating of the strand shell or nonuniform heat transfer along the peripheral line of the strand cross-section.
The corners of the die cavity of tubular casting dies are rounded by concave surfaces. The larger the concave surfaces in the die cavity are made, the more difficult it is to achieve uniform cooling between a strand shell being formed and the casting die walls, in particular over the periphery of the die cavity. The onset of strand solidification just beneath the bath level in the casting die proceeds differently on the straight portions of the die cavity periphery than in the concave surface regions. Heat flow at the straight or substantially straight portions is virtually one-dimensional and obeys the law governing heat transmission through a planar wall. In contrast, heat flow in the rounded corner regions is two-dimensional and obeys the law governing heat transmission through a curved wall.
As it forms, the strand shell is in general initially thicker in the corner regions than on the straight surfaces and begins to shrink earlier and to a greater extent. This means that after only approx. 2 seconds, the strand shell draws away from the die wall in the corner regions and an air gap forms which severely impairs heat transmission. This impairment of heat transmission not only delays further shell growth, but may even result in remelting of already solidified interior layers of the strand shell. This fluctuation in the heat flow (cooling and reheating) leads to strand defects such as superficial and internal lengthwise cracks at the edges or in regions close to the edges, and to defects in shape such as rhomboid deformation, necking etc.
The larger the concave surfaces are made relative to the side length of the strand cross-section, in particular if the radii of the concave surfaces account for 10% and more of the side length of the die cavity cross-section, the greater will be the incidence and extent of the stated strand defects. This is one reason why the concave surface radii are generally limited to 5 to 8 mm, although greater levels of rounding at the strand edges would be advantageous for subsequent rolling.
JP-A-53 011124 discloses a billet casting die for continuous casting with corner radii rounded as concave surfaces. The strand may cool irregularly in such casting dies and strands may be obtained with a diamond-shaped cross-section and corresponding edge defects, such as cracks etc. In order to avoid such strand defects, said document proposes equipping a rectangular casting die cavity with 2 small and 2 large concave corner surfaces. Using these different corner radii of the concave surfaces, it is intended to effect solidification of a strand shell of irregular thickness. It is intended to compensate the delayed solidification in the corners with large radii by enhanced edge cooling in the secondary cooling zone immediately on discharge from the casting die. These measures are intended to result in an unwarped strand cross-section.
JP-A-60 040647 discloses a continuous casting die for a blank. When casting blanks, lengthwise cracks often occur at the transition from the central web to the two end flanges. In the casting die, this transitional part is a convexly rounded edge portion onto which the profile strand shrinks slightly on cooling of the central web. In order to avoid this shrinkage or the formation of cracks, said document proposes providing this convex transitional curve of the casting die with a continuously increasing curvature towards the central web.
JP-A-11 151555 discloses a further casting die for continuously casting billets and blooms. In order to avoid rhombic distortion of the strand cross-section in this casting die too and additionally to increase casting speed, the casting die is provided with specially shaped corner cooling parts at the four corners which are provided with concave surfaces. At the pouring end, these corner cooling parts are circular recesses in the die wall which diminish in the direction of strand travel and, towards the die outlet, reduce to the rounding of the concave corner surface. The degree of curvature of the circular recess increases in the direction of strand travel towards the die outlet. This shape is intended to ensure uninterrupted contact between the corner region of the strand shell and the corner parts of the casting die.